The present invention relates to a semiconductor integrated circuit, and more particularly to a semiconductor integrated circuit and a power supply circuit therefor which employ a constant current source circuit to reduce dependence on the power supply and dependence on the temperature characteristics of the integrated circuit.
Generally, the characteristics of integrated circuits vary depending on environmental temperature, power supply, process variations and so on. A power supply circuit may be used in order to reduce the dependence of the characteristics of the integrated circuit on these parameters. The characteristics of a particular integrated circuit are defined by a certain power supply range and a temperature range so that a power supply circuit having constant characteristics within those ranges may be provided to stabilize the characteristics of the integrated circuit.
One such example is an input/output circuit of an ECL (Emitter Coupled Logic) memory integrated circuit. The standard about the input/output of integrated circuits called 100k ECL defines an input/output range for predetermined temperature and power supply ranges and has been realized in the prior art by employing a bandgap voltage reference circuit as described below.
Conventional bandgap reference circuits are discussed in IEEE, Journal of Solid-State Circuits, VOL 26, NUMBER 1 (1991), pp 77-80; IEEE, Journal of Solid-State Circuits, VOL 22, NUMBER 1 (1987), pp 71-76; and "Analog Integrated Circuit Design Techniques, Book One", (1990, published by Baihukan) pp 270-276 (Japanese Version of "Analysis and Design of Analog Integrated Circuits" by John Wiley and Sons, Inc., New York. 1984).
The bandgap reference circuit in general has a VT generation section and a VBE generation section and utilizes that the voltages generated from these two sections have dependence of opposite polarities to each other on temperature to provide a voltage output free of the temperature dependence.
VT designates a voltage expressed by kT/q and is called "thermal voltage". The magnitude of VT has positive dependence (positive temperature coefficient) on absolute temperature T. VBE designates forward voltage generated between the base and the emitter of a bipolar transistor, and its magnitude has negative temperature dependence (negative temperature coefficient) and generally ranges from 0.6 volts to 0.8 volts. The bandgap reference circuit multiplies these two voltages VT and VBE with appropriate coefficients, respectively, and adds them to provide an output voltage free of the temperature dependence.
Generally, the voltage VT is generated by the following method. Specifically, since the difference between the VBE voltages of two bipolar transistors is proportional to VT, a voltage proportional to VT is generated by applying a resistive element with a difference voltage of VBE of the bipolar transistors.
A conventional circuit employing a bandgap reference circuit is described, for example, in IEEE, Journal of Solid-State Circuits, VOL. 22, NUMBER 1 (1987), pp 72. VBB designates a voltage reference based On VCC. This is used for a voltage reference for determining an input logic threshold level in ECL LSI's. This voltage is compensated for the temperature dependence and the power supply dependence such that the voltage value does not vary with fluctuations in temperature and power supply voltage.
However, the constant voltage generation circuit of the prior art described above has a drawback that the operation is disabled in a low voltage range.
FIG. 15 shows a conventional bandgap reference circuit. A difference voltage between base-to-emitter voltages of a pair of bipolar transistors, which present a constant collector current ratio, is proportional to a thermal voltage VT. Therefore, the difference voltage of VBE between bipolar transistors Q1 and Q2 is proportional to absolute temperature and is applied to a resistive element R2. Thus, a current proportional to absolute temperature flows through R2.
Here, bipolar transistors Q13, Q14, and resistive elements R16, R15 are circuit elements for setting a ratio of currents which flow through the bipolar transistors Q1, Q2, respectively.
Thus, a voltage proportional to absolute temperature is generated across a resistive element R14, which is added to a base-to-emitter voltage of a bipolar transistor Q8 to provide a voltage VBB free of the temperature dependence.
In the drawing, VCC designates a high voltage side power supply, and VEE a low voltage side power supply.
A resistive element R13 and a bipolar transistor Q12 set base voltages of the bipolar transistors Q13 and Q14 and also set a collector voltage of the bipolar transistor Q2.
For the ease of understanding, the circuit arrangement described above is illustrated in block form in FIG. 16.
In the drawing, an I.varies.kT/q generation section corresponds to a circuit portion comprising the bipolar transistors Q1, Q2 and the resistive element R2 in FIG. 15. A current ratio setting circuit block in the drawing corresponds to the bipolar transistors Q14, Q13 and the resistive elements R15, R16 in FIG. 15. A V.varies.kT/q generation section corresponds to R14 in FIG. 15.
Since the conventional bandgap reference circuit arranges these three circuit blocks in series between the high voltage side power supply and the low voltage side power supply, the sum of respective minimum voltages necessary to operate the three circuit blocks is required as a power supply voltage between the high voltage side power supply and the low voltage side power supply in order to enable the whole bandgap reference circuit.